1. Field of the Invention
The present invention generally relates to a method and an apparatus for increasing the productivity of an existing well. More particularly, the invention relates to treating a portion of the existing well to stimulate production.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface of the well to a designated depth. An annular area is thus defined between the outside of the casing and the earth's formation. This annular area is filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore.
Historically, wells have been drilled with a column of fluid in the wellbore designed to overcome any formation pressure encountered as the wellbore is formed. This “overbalanced condition” restricts the influx of formation fluids such as oil, gas. or water into the wellbore. Typically, well control is maintained by using a drilling fluid with a predetermined density to maintain a hydrostatic pressure in the wellbore at a higher pressure than a formation pressure. In the overbalanced condition, formation damage may occur as the hydrostatic pressure forces the drill cuttings, and “fines” into the formation. Additional damage occurs if the drilling fluid flows into the formation. This flow of fluid into the formation can cause pores in the formation to become obstructed with drilling fluid and associated particulate matter. That obstruction can decrease formation permeability. Additionally, the cuttings or other solids form a wellbore “skin” along the interface between the wellbore and the formation. The wellbore skin restricts the flow of the formation fluid and thereby damages the well.
One method of addressing the damage to the wellbore or the lowered productivity of the well as described above is with some form of hydraulic fracturing treatment such as an “acid frac” operation. In the acid frac operation, an acid, such as hydrochloric acid, is used in a formation to etch open faces of induced fractures and natural fractures. When the treatment is complete, the fracture closes and the etch surfaces provide a high conductivity path from the formation to the wellbore. In some situations, small sized particles are mixed with fracturing fluid to hold fractures open after the hydraulic fracturing treatment. This is known in the industry as prop and frac. In addition to the naturally occurring sand grains, man made or specially engineered proppants, such as resin coated sand or high strength ceramic material, may also be used to form the fracturing mixture used to “prop and frac”. Proppant materials are carefully sorted for size and sphericity to provide an effective means to prop open the fractures, thereby allowing fluid from the formation to enter the wellbore.
The hydraulic fracturing treatment may be employed both in a wellbore lined with casing and an open hole wellbore. Generally, if the wellbore is lined with casing, a perforating gun is used prior the fracturing treatment to form a fluid path between the formation and the interior of the wellbore. The perforating gun is a device used to perforate the casing of an oil or gas well at an area of interest. Preferably, the perforating gun is located at a desired location adjacent a formation and then is activated by triggering a series of explosive charges to perforate the casing, thereby forming the fluid path between the formation and the casing. Thereafter, the perforating gun is typically moved to another area of interest where treatment is desired and subsequently removed from the wellbore after each area of interest is perforated.
After the fluid path between the formation and the casing is established, fracturing fluid, such as a specially engineered fluid, is pumped at high pressure and rate into the formation being treated, thereby causing the fracture to open. For example, the wings of a vertical fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. As previously discussed, proppants, such as grains of sand of a particular size, are mixed with the fracturing fluid to keep the fracture open after the treatment is complete. In this manner, hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area and increases productivity.
One problem associated with using the hydraulic fracturing treatment relates to damaging the treated area after the hydraulic fracturing treatment is complete. For instance, the vertical portion of the wellbore is typically filled with fluid to maintain well control before the fracturing equipment is removed from the wellbore. However, the fluid in the vertical portion creates a hydrostatic head due to the density of the fluid which will typically force existing wellbore fluid into the newly formed fractures and thus “killing” the well by stopping the flow of formation fluid or by restricting the formation fluid flow into the wellbore. Another problem arises due to the cost of the operation. For instance, the fracturing fluid is expensive and the volume required to treat a wellbore creates logistical issues to achieve the desired result. Additionally, the cost is magnified when the hydraulic fracturing treatment is conducted on a deep wellbore. In this situation, jointed pipe is typically required in conjunction with the coiled tubing to reach the area of interest in the deep wellbore. By deploying jointed pipe in the wellbore, additional costly equipment is required to maintain well control, such as a snubbing unit which is well known in the art. Furthermore, another problem associated with using the hydraulic fracturing treatment is related to the degree of control of limiting the treatment to a selected region of the wellbore. It is often difficult for the operator to ensure that the fracturing fluid is only used to treat the selected region of the wellbore.
There is a need, therefore, for controlling the hydrostatic head in the wellbore to prevent the killing of the well upon the completion of the hydraulic fracturing treatment. There is a further need for a method for limiting the treatment to a specific region of the wellbore. There is yet a further need for a cost effective method to increase the productivity of an existing well.